Fault tolerant implantable medical system

ABSTRACT

The disclosure describes implantable medical systems that respond to occurrence of a lead-related condition by utilizing an elongated coil electrode in defining an alternative pacing therapy vector to maintain optimal drain of an IMD power supply. An exemplary system includes a medical electrical lead having an elongated electrode and an improved sensing and therapy delivery circuitry to provide the alternative pacing therapy vector responsive to the lead-related conditions. The system reconfigures the operation of the sensing and therapy delivery circuitry triggered by the switch to the alternative pacing therapy vector.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.13/457,884, filed Apr. 27, 2012 and entitled “FAULT TOLERANT IMPLANTABLEMEDICAL SYSTEM”, now U.S. Pat. No. 9,079,037, which is hereinincorporated by reference in its entirety. Reference is made to thecommonly-assigned U.S. Pat. No. 8,996,111, entitled “LEAD RECOGNITIONFOR AN IMPLANTABLE MEDICAL SYSTEM”, which is herein incorporated byreference in its entirety.

FIELD

The disclosure relates generally to an implantable medical system. Inparticular, the disclosure relates to alternate therapy vectors forproviding a therapy function in the event of a lead-related conditionassociated with the lead system.

BACKGROUND

A wide variety of implanted medical devices (IMDs) for delivering atherapy or monitoring a physiologic condition which can employ one ormore elongated electrical leads and/or sensors are available. Such IMDscan monitor or deliver therapy to the heart, muscle, nerve, brain, andstomach or other organs. Examples of such IMDs include implantablecardioverter defibrillator devices, which have a pulse generator and oneor more electrical leads with one or more electrodes that conductsignals to and receive signals from the patient's heart. Theseelectrical lead(s) and their electrode(s) are placed in or proximate tothe organ such that an electrical signal between the electrodes iscapable of stimulating the organ. The electrodes may be configuredeither to deliver a stimulus to the organ, or to detect or sense anintrinsic electrical event associated with the organ.

The leads associated with IMDs typically include a lead body extendingbetween a proximal lead end and a distal lead end that incorporates theone or more exposed electrode or sensor elements located at or near thedistal lead end. One or more elongated electrical conductors extendthrough the lead body from a connector assembly provided at a proximallead end for connection with associated IMD to the electrode or sensorelement located at the distal lead end or along a section of the leadbody. Each electrical conductor is typically electrically isolated fromother electrical conductors and is encased within an outer sheathinsulator, which electrically insulates the lead conductors from bodytissue and fluids.

Consideration is taken of various stresses that may be applied to thelead body during an implantation, a lead repositioning procedure, orchronic implanted stresses. For example, continuous flexing of thecardiac lead bodies due to the beating of the heart is an importantconsideration in maintaining the lead's structural integrity. Theeffects of lead body damage can progress from an intermittentmanifestation to a more continuous effect. In extreme cases, insulationof one or more of the electrical conductors can be breached, causing theconductors to contact one another or body fluids resulting in a lowimpedance or short circuit. In other cases, a lead conductor canfracture and exhibit an intermittent or continuous open circuitresulting in an intermittent or continuous high impedance. Such leadissues resulting in short or open circuits, for example, can be referredto, for simplicity, as “lead-related conditions.”

In the case of cardiac leads, the ability to sense cardiac activityconditions accurately through a lead can be impaired by theselead-related conditions. Complete lead breakage impedes any sensingfunctions while lead conductor fractures or intermittent contact candemonstrate electrical noise that interferes with accurate sensing.During cardiac pacing or defibrillation, lead-related conditions canreduce the effectiveness of a pacing or defibrillation pulse or therapyto below that which is sufficient to pace or defibrillate the heart.

It is generally desirable to provide mechanisms to sustain therapydelivery in the event of a lead-related condition.

SUMMARY

Generally, the present disclosure addresses the need to provide back-uppacing and sensing vectors in response to a lead-related conditionimpacting one or more components of an implantable medical system, suchas a medical electrical lead. The ability to sustain therapy delivery inresponse to the occurrence of the lead-related condition is based inpart on the viability of the remaining therapy vectors. The remainingtherapy vectors are defined by the electrode-conductor sets that areunaffected by the lead-related condition.

In an embodiment, an implantable medical system for sustaining therapyincludes a lead having a first electrode and an alternate electrodewherein the system switches delivery of therapy from the first electrodeto the alternate electrode in response to a lead-related condition. Thesystem may further include an oversense reduction module thatreconfigures the therapy to be delivered triggered by the switch fromthe first electrode to the alternate electrode.

In another aspect, a sensing function of the implantable medical systemis reconfigured in response to occurrence of the lead-related condition.In one embodiment, the implantable medical system includes an oversensereduction module for processing of signals sensed through the alternateelectrode.

According to another illustrative embodiment, a method for deliveringtherapy includes providing a therapy through a first electrode;reconfiguring the therapy in response to a detected lead-relatedcondition and providing the therapy through a distal portion of analternate electrode. In another embodiment, the method may includereconfiguration of a sensing function in response to the occurrence ofthe lead-related condition for sensing through an alternate electrode.

The foregoing has outlined rather broadly certain features and/ortechnical advantages in order that the detailed description that followsmay be better understood. Additional features and/or advantages will bedescribed hereinafter which form the subject of the claims. It should beappreciated by those skilled in the art that the conception and specificembodiment disclosed may be readily utilized as a basis for modifying ordesigning other structures for carrying out the same purposes. It shouldalso be realized by those skilled in the art that such equivalentconstructions do not depart from the spirit and scope of the appendedclaims. The novel features, both as to organization and method ofoperation, together with further objects and advantages will be betterunderstood from the following description when considered in connectionwith the accompanying figures. It is to be expressly understood,however, that each of the figures is provided for the purpose ofillustration and description only and is not intended as a definition ofthe limits of the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exemplary implantable therapy system that may be used toprovide therapy to heart of a patient is described;

FIG. 2 is a conceptual diagram illustrating an implantable medicaldevice and leads of a therapy system in greater detail;

FIG. 3 is a conceptual diagram illustrating another example of therapysystem, which is similar to therapy system of FIGS. 1-2;

FIG. 4 is a functional block diagram of one example configuration of animplantable medical device;

FIGS. 5A-B depict alternate embodiments of an elongated electrode thatprovides an alternate pacing vector in the event of occurrence of alead-related condition;

FIG. 6 illustrates a block diagram of oversense reduction module forsensing cardiac events in accordance with an embodiment of thedisclosure;

FIG. 7 illustrates a flowchart that represents one embodiment of analgorithm that may be implemented in a comparative module for evaluationof the signals received from the elongated electrode;

FIG. 8 illustrates a process for sustaining therapy delivery in responseto occurrence of a lead-related condition according to one embodiment ofthe disclosure;

FIG. 9 illustrates a flow chart in accordance with an embodiment forassessing whether a lead system includes an elongate electrode of thetype contemplated in this disclosure; and

FIG. 10 depicts one example of the sub-threshold voltage pulse waveformgenerated by a stimulation generator of the implantable medical device.

DETAILED DESCRIPTION

The present disclosure can be practiced in the context of theimplantable medical systems described herein. In the interest ofclarity, not all features of an actual implementation are described inthis specification. It will of course be appreciated that in thedevelopment of any such actual embodiment, numerousimplementation-specific decisions must be made to achieve thedevelopers' specific goals, such as compliance with system-related andbusiness-related constraints, which will vary from one implementation toanother. Moreover, it will be appreciated that such a development effortmight be complex and time-consuming, but would nevertheless be a routineundertaking for those of ordinary skill in the art having the benefit ofthis disclosure.

An implantable cardiac system is used to sense electrical activityindicating events occurring in one or more heart chamber of a patient.The sensed electrical signals are transmitted in a raw form through oneor more lead(s) for processing by an implantable medical device (IMD) ofthe cardiac system. The sensed electrical activity is processed by theIMD and the results of the processing are used for controlling therapydelivery by the cardiac system. One aspect of such cardiac systems isthe difficulty of sustaining therapy in the presence of a conditionimpacting the lead's performance.

Generally, lead-related conditions may be understood to refer to anycondition prohibiting or frustrating use of the lead in the desiredmanner during normal operation of the cardiac rhythm management system.In addition to those already discussed, these conditions also includebut are not limited to parameters associated with physical conditions ofthe lead such as sensed noise, lead impedance outside a predeterminedrange, capture failure, capture amplitude voltage outside apredetermined range, intrinsic amplitude outside a predetermined range,failure to detect an expected event, and an electrical hardware failure.

Referring to FIG. 1, an exemplary implantable cardiac system that may beused to provide therapy to heart 12 of patient 14 is described. Patient14 ordinarily, but not necessarily, will be a human. Therapy system 10includes IMD 16, which is coupled to leads 18, 20, and 22, andprogrammer 24. IMD 16 may be, for example, an implantable pacemaker,cardioverter, and/or defibrillator that provides electrical signals toheart 12 via electrodes coupled to one or more of leads 18, 20, and 22.Each of leads 18, 20 and 22 may carry one or a set of electrodes. Theelectrode may extend about the circumference of each of leads 18, 20,and 22 and is positioned at a respective axial position along the lengthof each of the lead 18, 20, and 22.

Leads 18, 20, 22 extend into the heart 12 of patient 14 to senseelectrical activity of heart 12 and/or deliver electrical stimulation toheart 12. In the example shown in FIG. 1, right ventricular (RV) lead 18extends through one or more veins (not shown), the superior vena cava(not shown), and right atrium 26, and into right ventricle 28. Leftventricular (LV) coronary sinus lead 20 extends through one or moreveins, the vena cava, right atrium 26, and into the coronary sinus 30 toa region adjacent to the free wall of left ventricle 32 of heart 12. Inalternative embodiments, the LV lead 20 may also be introduced into theleft ventricle through the septal wall. Right atrial (RA) lead 22extends through one or more veins and the vena cava, and into the rightatrium 26 of heart 12.

IMD 16 may sense electrical signals attendant to the depolarization andrepolarization of heart 12 via electrodes (not shown in FIG. 1) coupledto at least one of the leads 18, 20, 22. In some examples, IMD 16provides pacing pulses to heart 12 based on the electrical signalssensed within heart 12. The configurations of electrodes used by IMD 16for sensing and pacing may be unipolar or bipolar. IMD 16 may alsoprovide defibrillation therapy and/or cardioversion therapy viaelectrodes located on at least one of the leads 18, 20, 22. IMD 16 maydetect arrhythmia of heart 12, such as fibrillation of ventricles 28 and32, and deliver defibrillation therapy to heart 12 in the form ofelectrical pulses. In some examples, IMD 16 may be programmed to delivera progression of therapies, e.g., pulses with increasing energy levels,until a fibrillation of heart 12 is stopped. IMD 16 detects fibrillationemploying one or more fibrillation detection techniques known in theart.

In some examples, programmer 24 may be a handheld computing device or acomputer workstation. Programmer 24 may include a user interface thatreceives input from a user. The user interface may include, for example,a keypad and a display, which may for example, be a cathode ray tube(CRT) display, a liquid crystal display (LCD) or light emitting diode(LED) display. The keypad may take the form of an alphanumeric keypad ora reduced set of keys associated with particular functions. Programmer24 can additionally or alternatively include a peripheral pointingdevice, such as a mouse, via which a user may interact with the userinterface. In some embodiments, a display of programmer 24 may include atouch screen display, and a user may interact with programmer 24 via thedisplay.

A user, such as a patient, physician, technician, or other clinician,may interact with programmer 24 to communicate with IMD 16. For example,the user may interact with programmer 24 to retrieve physiological ordiagnostic information from IMD 16. A user may also interact withprogrammer 24 to program IMD 16, e.g., select values for operationalparameters of the IMD.

For example, the user may use programmer 24 to retrieve information fromIMD 16 regarding the rhythm of heart 12, trends therein over time, ortachyarrhythmia episodes. As another example, the user may useprogrammer 24 to retrieve information from IMD 16 regarding other sensedphysiological parameters of patient 14, such as intracardiac orintravascular pressure, activity, posture, respiration, or thoracicimpedance. As another example, the user may use programmer 24 toretrieve information from IMD 16 regarding the performance or integrityof IMD 16 or other components of system 10, such as leads 18, 20, and22, or a power source of IMD 16.

The user may use programmer 24 to program a therapy progression, selectelectrodes used to deliver defibrillation shocks, select waveforms forthe defibrillation shock, or select or configure a fibrillationdetection algorithm for IMD 16. The user may also use programmer 24 toprogram aspects of other therapies provided by IMD 16, such ascardioversion or pacing therapies. In some examples, the user mayactivate certain features of IMD 16 by entering a single command viaprogrammer 24, such as depression of a single key or combination of keysof a keypad or a single point-and-select action with a pointing device.

IMD 16 and programmer 24 may communicate via wireless communicationusing any techniques known in the art. Examples of communicationtechniques may include, for example, low frequency or radiofrequency(RF) telemetry, but other techniques are also contemplated. In someexamples, programmer 24 may include a programming head that may beplaced proximate to the patient's body near the IMD 16 implant site inorder to improve the quality or security of communication between IMD 16and programmer 24.

FIG. 2 is a conceptual diagram illustrating IMD 16 and leads 18, 20, 22of therapy system 10 in greater detail. Leads 18, 20, 22 may beelectrically coupled to a stimulation generator, a sensing module, orother modules of IMD 16 via connector block 34. In some examples,proximal ends of leads 18, 20, 22 may include electrical contacts thatelectrically couple to respective electrical contacts within connectorblock 34. In addition, in some examples, leads 18, 20, 22 may bemechanically coupled to connector block 34 with the aid of set screws,connection pins or another suitable mechanical coupling mechanism.

Each of the leads 18, 20, 22 includes an elongated insulative lead body,which may carry a number of concentric coiled conductors, or parallelcable conductors in a multi-lumen lead body or co-radial conductors allof which are separated from one another by tubular insulative sheaths.In the illustrated example, a pressure sensor 38 and bipolar electrodes40 and 42 are located proximate to a distal end of lead 18. In addition,bipolar electrodes 44 and 46 are located proximate to a distal end oflead 20 and bipolar electrodes 48 and 50 are located proximate to adistal end of lead 22. In FIG. 2, pressure sensor 38 is disposed inright ventricle 28. Pressure sensor 30 may respond to an absolutepressure inside right ventricle 28, and may be, for example, acapacitive or piezoelectric absolute pressure sensor. In other examples,pressure sensor 30 may be positioned within other regions of heart 12and may monitor pressure within one or more of the other regions ofheart 12, or may be positioned elsewhere within or proximate to thecardiovascular system of patient 14 to monitor cardiovascular pressureassociated with mechanical contraction of the heart.

Among the electrodes, some of the electrodes may be provided in the formof coiled electrodes that form a helix, while other electrodes may beprovided in different forms. Further, some of the electrodes may beprovided in the form of tubular electrode sub-assemblies that can bepre-fabricated and positioned over the body of leads 18, 20, 22, wherethey are attached and where electrical connections with conductiveelements within the leads 18, 20, 22 can be made. For example,electrodes 40, 44 and 48 may take the form of ring electrodes, andelectrodes 42, 46 and 50 may take the form of extendable helix tipelectrodes mounted retractably within insulative electrode heads 52, 54and 56, respectively. Each of the electrodes 40, 42, 44, 46, 48 and 50may be electrically coupled to a respective one of the coiled conductorswithin the lead body of its associated lead 18, 20, 22, and therebycoupled to respective ones of the electrical contacts on the proximalend of leads 18, 20 and 22.

Electrodes 40, 42, 44, 46, 48 and 50 may sense electrical signalsattendant to the depolarization and repolarization of heart 12. Theelectrical signals are conducted to IMD 16 via the respective leads 18,20, 22. In some examples, IMD 16 also delivers pacing pulses viaelectrodes 40, 42, 44, 46, 48 and 50 to cause depolarization of cardiactissue of heart 12. In some examples, as illustrated in FIG. 2, IMD 16includes one or more housing electrodes, such as housing electrode 58,which may be formed integrally with an outer surface ofhermetically-sealed housing 60 of IMD 16 or otherwise coupled to housing60. In some examples, housing electrode 58 is defined by an uninsulatedportion of an outward facing portion of housing 60 of IMD 16. Otherdivision between insulated and uninsulated portions of housing 60 may beemployed to define one or more housing electrodes. In some examples,housing electrode 58 comprises substantially all of housing 60. Any ofthe electrodes 40, 42, 44, 46, 48 and 50 may be used for unipolarsensing or pacing in combination with housing electrode 58. As is knownin the art, housing 60 may enclose a stimulation generator thatgenerates cardiac pacing pulses and defibrillation or cardioversionshocks, as well as a sensing module for monitoring the patient's heartrhythm.

Leads 18, 20, 22 also include elongated electrodes 62, 64, 66,respectively, which may take the form of a coil. IMD 16 may deliverdefibrillation shocks to heart 12 via any combination of elongatedelectrodes 62, 64, 66, and housing electrode 58. Electrodes 58, 62, 64,66 may also be used to deliver cardioversion therapy to heart 12.Additionally, unipolar pacing and/or sensing may be implemented, forexample, using one of the coil electrodes 62, 64, 66 referenced to thecan electrode 58 in accordance with embodiments of this disclosure. Theelectrodes 62, 64, 66 may be fabricated as will be described in moredetail in FIG. 6.

Pressure sensor 38 may be coupled to one or more coiled conductorswithin lead 18. In FIG. 2, pressure sensor 38 is located more distallyon lead 18 than elongated electrode 62. In other examples, pressuresensor 38 may be positioned more proximally than elongated electrode 62,rather than distal to electrode 62. Further, pressure sensor 38 may becoupled to another one of the leads 20, 22 in other examples, or to alead other than leads 18, 20, 22 carrying stimulation and senseelectrodes.

The configuration of therapy system 10 illustrated in FIGS. 1 and 2 ismerely one example. In other examples, a therapy system may includeepicardial leads and/or patch electrodes instead of or in addition tothe transvenous leads 18, 20, 22 illustrated in FIG. 1. Further, IMD 16need not be implanted within patient 14. In examples in which IMD 16 isnot implanted in patient 14, IMD 16 may deliver defibrillation shocksand other therapies to heart 12 via percutaneous leads that extendthrough the skin of patient 14 to a variety of positions within oroutside of heart 12.

In other examples of therapy systems that provide electrical stimulationtherapy to heart 12, a therapy system may include any suitable number ofleads coupled to IMD 16, and each of the leads may extend to anylocation within or proximate to heart 12. For example, other examples oftherapy systems may include three transvenous leads located asillustrated in FIGS. 1 and 2, and an additional lead located within orproximate to left atrium 33. Other examples of therapy systems mayinclude a single lead that extends from IMD 16 into right atrium 26 orright ventricle 28, or two leads that extend into a respective one ofthe right ventricle 26 and right atrium 28. An example of this type oftherapy system is shown in FIG. 3.

FIG. 3 is a conceptual diagram illustrating another example of therapysystem 70, which is similar to therapy system 10 of FIGS. 1-2, butincludes two leads 18, 22, rather than three leads. Leads 18, 22 areimplanted within right ventricle 28 and right atrium 26, respectively.Therapy system 70 shown in FIG. 3 may be useful for providingdefibrillation and pacing pulses to heart 12.

FIG. 4 is a functional block diagram of one example configuration of IMD16, which includes processor 80, memory 82, stimulation generator 84,sensing module 86, telemetry module 88, and power source 90. Memory 82includes computer-readable instructions that, when executed by processor80, cause IMD 16 and processor 80 to perform various functionsattributed to IMD 16 and processor 80 herein. Memory 82 may include anyvolatile, non-volatile, magnetic, optical, or electrical media, such asa random access memory (RAM), read-only memory (ROM), non-volatile RAM(NVRAM), electrically-erasable programmable ROM (EEPROM), flash memory,or any other digital media.

Processor 80 controls stimulation generator 84 to deliver stimulationtherapy to heart 12 according to a selected one or more of therapyprograms, which may be stored in memory 82. Specifically, processor 44may control stimulation generator 84 to deliver electrical pulses withthe amplitudes, pulse widths, frequency, or electrode polaritiesspecified by the selected one or more therapy programs.

Stimulation generator 84 is electrically coupled to electrodes 40, 42,44, 46, 48, 50, 58, 62, 64, and 66, e.g., via conductors of therespective lead 18, 20, 22, or, in the case of housing electrode 58, viaan electrical conductor disposed within housing 60 of IMD 16.Stimulation generator 84 is configured to generate and deliverelectrical stimulation therapy to heart 12. For example, stimulationgenerator 84 may deliver defibrillation shocks to heart 12 via at leasttwo electrodes 58, 62, 64, 66. In some examples, stimulation generator84 delivers pacing, cardioversion, or defibrillation stimulation in theform of electrical pulses. In other examples, stimulation generator maydeliver one or more of these types of stimulation in the form of othersignals, such as sine waves, square waves, or other substantiallycontinuous time signals. In accordance with embodiments of thisdisclosure, stimulation generator 84 may deliver pacing pulses viaelongate electrodes 62, 64, and 66.

Stimulation generator 84 may include a switch module and processor 80may use the switch module to select, e.g., via a data/address bus, whichof the available electrodes are used to deliver defibrillation shocks orpacing pulses. The switch module may include a switch array, switchmatrix, multiplexer, or any other type of switching device suitable toselectively couple stimulation energy to selected electrodes.

Sensing module 86 monitors signals from at least one of electrodes 40,42, 44, 46, 48, 50, 58, 62, 64 or 66 in order to monitor electricalactivity of heart 12, e.g., via electrocardiogram (ECG) signals. Sensingmodule 86 may also include a switch module to select which of theavailable electrodes are used to sense the heart activity. As will bedescribed in more detail elsewhere, the sensing module 86 may employoversense reduction circuitry (FIG. 6) for signals sensed via electrodes62, 64, or 66. In some examples, processor 80 may select the electrodesthat function as sense electrodes via the switch module within sensingmodule 86, e.g., by providing signals via a data/address bus. In someexamples, sensing module 86 includes one or more sensing channels, eachof which may comprise an amplifier. In response to the signals fromprocessor 80, the switch module of sensing module 86 may couple theoutputs from the selected electrodes to one of the sensing channels.

In some examples, sensing module 86 may include one or more amplifiersthat receive signals from one or more of the electrodes. In someexamples, the amplifiers may take the form of an automatic gaincontrolled amplifier that provides an adjustable sensing threshold as afunction of the measured P-wave or R-wave amplitude of the heart rhythm.Examples of such amplifiers are described in U.S. Pat. No. 5,117,824 toKeimel et al., which issued on Jun. 2, 1992 and is entitled, “Apparatusfor Monitoring Electrical Physiologic Signals,” and is incorporatedherein by reference in its entirety. Furthermore, in some examples, oneor more of the sensing channels of sensing module 86 may be selectivelycoupled to housing electrode 58, with elongated electrodes 62, 64, or66, e.g., for unipolar sensing of R-waves or P-waves in response tolead-related conditions.

In some examples, sensing module 86 includes a channel that comprises anamplifier with a relatively wider pass band than the R-wave or P-waveamplifiers. Signals from the selected sensing electrodes that areselected for coupling to this wide-band amplifier may be provided to amultiplexer, and thereafter converted to multi-bit digital signals by ananalog-to-digital converter for storage in memory 82 as an electrogram(EGM). In some examples, the storage of such EGMs in memory 82 may beunder the control of a direct memory access circuit. Processor 80 mayemploy digital signal analysis techniques to characterize the digitizedsignals stored in memory 82 to detect and classify the patient's heartrhythm from the electrical signals. Processor 80 may detect and classifythe heart rhythm of patient 14 by employing any of the numerous signalprocessing methodologies known in the art.

In general, the ability to sustain therapy delivery in response to alead-related condition is limited by the availability of alternatetherapy vectors that are defined by the remaining, viable, electrodecombinations. For example, some cardiac systems have been provided withredundant electrodes or leads and such systems may revert to analternate electrode combination on the redundant lead for backup therapyfunctionality. However, providing redundancy in the lead system may notbe feasible or desirable.

FIGS. 5A-B depict alternate embodiments of an elongated electrode thatprovides an alternate pacing vector in the event of occurrence of alead-related condition. The elongated electrode includes a first sectionand a second section with the first section having a first material thatis different from a second material of the second. The elongatedelectrode defines a first vector for providing cardioversion and/ordefibrillation therapy and a second vector for alternative pacingtherapy. The construction of the elongated electrode first and secondsections with different materials facilitates selective pacing andsensing making the vector defined by the elongated electrode and thehousing electrode amenable as an alternative pacing vector.

The present inventors have noted the need to provide viable alternativepacing vectors in the presence of lead-related conditions withoutincreasing the complexity or size of the lead systems. In particular,the construction of conventional elongated (coil) electrodes and theconductors coupling these electrodes to the IMD renders them lesssusceptible to being compromised by the stresses that are applied to thelead system. For example, the conductor(s) associated with the elongatedelectrode is constructed to enable coupling of defibrillation energy.The defibrillation conductors are designed to withstand a greater amountof stress in comparative relation to the conductors associated withpacing electrodes. Yet, such coil electrodes are generally unsuitable asan alternative for providing a fault tolerant pacing vector. Incomparison to ring and tip electrodes, the coil electrodes generallyhave a much larger surface area. For example, the surface area of a coilelectrode may range from 50 to 100 times greater than the ring and tipelectrode. Owing to the larger surface area, the electric field isweakened, relative to the smaller surface area of the ring or tipelectrodes, because the current is spread over the entire surface area.The larger surface area of the coil electrode makes it impractical fordelivering pacing pulses because it requires that relatively greatercurrent be delivered to achieve capture in comparison to the energy thatis typically delivered through a tip or ring electrode for the sametherapeutic function. In most instances, the requirement for increasedcurrent consumption owing to the greater coil electrode surface areawould at worst result in failure to capture the heart and at best wouldresult in enhanced current drain thereby exhausting the batteryresources within a much shorter time frame in comparison to ring and tipelectrodes.

In FIG. 5A, the elongated electrode 62 depicted includes a first section102 that is located proximal to a distally located second section 104.The elongated electrodes 64 and 66 may also be constructed similar toelongated electrode 62. In one embodiment, the entire surface area ofelongated electrode 62 includes a first material and the distal, second,section 104 is further formed with a second material that encapsulatesthe first material over that portion. The resulting electrode 62therefore has the exposed surface area of the first portion 102 havingthe first material while the surface area of the exposed second portion104 has the second material.

Unlike the conventional coil electrodes, the elongated electrode 62 maybe formed such that only a selected one of the first and second sectionsconducts electric currents that are below a specified threshold. Thematerial of the selected section may be one that permits conduction ofcurrent that has a value at or below a certain threshold while thematerial of the unselected section would perform as an insulator whensuch currents are delivered. Both sections would, however, be conductivefor currents exceeding the threshold. Such an electrode configurationenables the electric field of certain magnitude, for example pacingpulses, to be focused only on the selected first or second section. Inother words, an optimal electric field sufficient for providing pacingtherapy that is applied to the electrode would be focused only on thesurface area of the conductive region of the elongated electrode.

The proportions of the proximal portion 102 to the distal portion 104are chosen to facilitate delivery of pacing stimulus through the distalportion 104 but to permit delivery of defibrillation therapy throughboth the proximal portion 102 and the distal portion 104. Theproportions of the distal to proximal portions are selected based on thetotal surface area of a given electrode 62 that delivers a definedamount of energy. For example, a typical elongated electrode 62 having atotal surface area of approximately 200 mm² will have the distal portion104 occupying about 20% of the total surface area or about 40 mm². Inanother example, an elongated electrode with a surface area of about 400mm² will have include a proximal portion 102 that occupies about 90% ofthe total electrode surface area and a distal portion with a surfacearea of about 40 mm². In other words, for the typical elongatedelectrode having a surface area ranging between 200 to 600 mm², thedistal portion will range from about 20 to 40 mm², respectively. Inother embodiments, the relative surface areas of the proximal portion todistal portion may be expressed as a ratio ranging from about 10:90 toabout 99:1, respectively.

FIG. 5B illustrates an alternative embodiment of an elongated electrode.In the alternative embodiment, the electrode 62 b may be constructedsuch that the second section is segmented into multiple portions thatare interspersed between the first section. As such, rather than havinga continuous first section coupled to a continuous second section as isillustrated in the embodiment of FIG. 5A, the alternate embodiment ofFIG. 5B illustrates the electrode 62 formed with the first section 102being distributed over the first section. The resulting configuration isthe electrode 62 b that has multiple second sections 104 a, 104 b, 104c, and 104 d that are interspersed between multiple first sections 102a, 102 b, 102 c, 102 d, and 102 e. Such an elongated electrodeconfiguration may be beneficial in providing an elongated electrode thatprovides closer contact with cardiac tissue for example when theelectrode is bent towards the blood pool. For example, the surface areaof the second section 104 may be divided into four sections 104 a-d,which combine to equate to the total surface area of the distal portion104 and conversely, the sections 102 a-e all equate to the total surfacearea of the proximal portion 102.

Examples of the first material include (used in their native oxide formor surface treated, i.e., anodized, doped, ion implanted, reactivesputtered, or any other chemical or physical treatment of the surface)include valve metals such as titanium, tungsten, chromium, aluminum,zirconium, hafnium, zinc, vanadium, niobium, tantalum, bismuth,antimony, and also include oxides, mixtures, and alloys thereof. Othermaterials that can be used include diamond, diamond-like-carbon (DLC)and other nanostructured materials, metal oxides or mixtures of metaloxides, nitrides, carbides, semiconductors, conductive ceramics andceramic oxides, conductive glasses, conductive polymers, gels,polymer-metal composites, and ceramic or glass composites. These variousmaterials may substitute for each other or may be used in combination asthe first material. The materials may be further provided with an oxidecoating (e.g., tantalum pentoxide “Ta₂O₅”) which imparts usefulproperties such as corrosion resistance, EMI (electromagneticinterference) isolation and electrical resistance.

Examples of the second material include good conductor metals such asplatinum, rhenium, vanadium, zirconium, palladium, iridium, titanium,niobium, tantalum, ruthenium, silver, molybdenum, silver chloride,cobalt, chromium, tungsten, magnesium, manganese, and their alloys.Examples of the second material also include conductive diamonds,nanotubes, and other nanostructured materials, nonmetals such as carbon,nitrides, conductive polymers, conductive ceramics and composites madeof combinations of these materials, including combinations of metals andnonmetals. The nonmetals may also be combined with the metals of thesecond material to form the second metal.

FIG. 6 illustrates a block diagram of oversense reduction module 200 forsensing cardiac events in accordance with an embodiment of thedisclosure. Module 200 may be incorporated or coupled to conventionalsense circuitry such as sensing module 86 of IMD 16. The oversensereduction module 200 deconstructs the composite signal into theindividual first and second constituent signals that are received fromthe proximal and distal portions of the elongated electrode in responseto a cardiac event. The deconstruction of the sensed signal with module200 compensates for the difference in the propagation of a single eventsensed by both the proximal and distal portions of the elongatedelectrode 62, for example.

The oversense reduction module 200 includes a filtering module 202 thatisolates the two constituent signals of the composite signal. Thefiltering characteristics of filtering module 202 may be selected basedon the material properties for the distal and proximal portions ofelongated electrode 62. In particular, the module 202 includes aplurality of band pass filters that are tuned to deconstruct thecomposite signal into its constituent signals. For example, in theembodiment of the electrode having two portions, i.e., a distal and aproximal portion, the module 202 will have two band pass filters thatwill each output the signals sensed through the distal and proximalportions. The input of these band pass filters is the composite signalsensed by the elongated electrode 62. Because the characteristics of thematerials are known, each of the band pass filters can be designed topass through the frequency associated with one material whileattenuating the frequency associated with the other material. The rawsignal from the distal portion may be provided to the sense circuitry inaccordance with conventional sensed signal processing.

An additional aspect of the oversense reduction module 200 may be acomparative analysis of the deconstructed signals. This analysis may beperformed in comparative module 204. The comparative module 204 mayutilize an algorithm to determine whether the two constituent signalsthat are obtained from the composite signal sensed via the elongatedelectrode actually represent a single cardiac event. The comparativemodule 204 may employ predetermined criteria to determine the instancesin which to evaluate signals that may be suspected to represent the sameevent but otherwise appear as discrete signals due to disparities owingto the sensing by the elongated electrode. Such predetermined criteriamay include the interval between the constituent signals of thecomposite signal. For example, if the interval between the signals isgreater than 100 milliseconds, the comparative module 204 will deem thecomposite signal as requiring further evaluation. In one embodiment, thealgorithm may evaluate the suspect signals by comparing theirmorphologies to determine whether there is a correlation between thesignals. In response to determining that there is a correlation, one ofthe two signals such as the proximal signal may be discarded fromfurther processing. For example, the lagging signal may be discarded asbeing a redundant signal. In response to ascertaining that the signal isindeed a potential cardiac event, the sensed signal is propagated to thesense circuit for further processing.

FIG. 7 illustrates a flowchart that represents one embodiment of analgorithm that may be implemented in comparative module 204 forevaluation of the signals received from the elongated electrode.Although this example embodiment is described with reference to theelectrode 62 illustrated in FIG. 5A, the concepts can easily be appliedto the signals received from the segments of first and second sectionsof the electrode of FIG. 5B. As previously discussed, filtering isapplied to isolate a signal A that represents the signal received fromthe proximal portion 102 of the elongated electrode from a signal Brepresenting the signal received from the distal portion 104 of theelongated electrode. The signal A may be stored in a memory location atstep 302 a. Additionally, the signal B may be stored in a memorylocation at step 302 b. The signals A and B may be converted from theirraw analog form into digital signals at steps 304 a and 304 b,respectively.

The inventors have observed that one resultant effect of sensingphysiologic signals via the proximal and distal portions is that thesignal A exhibits a time shift relative to signal B. To account for thetime shift, time markers may be applied at steps 306 a, 306 b to thesignals A and B, respectively, to accurately depict the interval betweenthe signals.

The signals may subsequently be analyzed at 308 to determine whether thereceived composite signal is indicative of a cardiac event. For example,signals A and B may be compared to determine whether they are identical.The comparison may utilize the time markers to accurately overlay thesignals. In another example, one or both signals may be compared to areference signal to determine whether the received signal is consistentwith a cardiac event. Such a reference signal may be the morphology ofthe patient's known cardiac waveform, for example. Based on the resultsof the analysis, the comparative module 204 determines (310) whether thereceived signals are either representative of a potential sensed eventor a noise signal. The signal is subsequently identified as either asensed event (312) or a noise signal (314).

The flowchart of FIG. 8 illustrates a process for sustaining therapydelivery in response to occurrence of a lead-related condition accordingto one embodiment of the disclosure. For ease of discussion and withoutintending to be limiting, the process is described as being embodied inthe system 10. Therapy delivery is sustained utilizing an electrodecombination that includes an elongated electrode to define an alternatepacing vector upon the occurrence of a lead-related condition.

Typically, pacing energy is delivered to the heart tissue via a cathodeelectrode(s) at one or more pacing sites, with a return path providedvia an anode electrode(s). If capture occurs, the energy injected at thecathode electrode site creates a propagating wavefront of depolarizationwhich may combine with other depolarization wavefronts to trigger acontraction of the cardiac muscle. The cathode and anode electrodecombination that delivers the pacing energy defines the pacing vectorused for pacing. The position of the cathode relative to cardiac tissuecan be used to define an electrode combination and/or a pacing site.

The process utilizes a primary vector for delivery of the pacing therapyto the patient (400). The primary vector is defined for an “electrodecombination,” where the term “electrode combination” denotes that atleast one cathode electrode and at least one anode electrode are used.For example, multiple electrodes that are electrically connected may beused as the anode and/or multiple electrodes that are electricallyconnected may be used as the cathode and the pacing pulses are deliveredvia the cathode/anode electrode combinations.

Examples of such pacing electrodes are the ring and tip electrodes, suchas 40, 42, 44, 46, 48 and 50. Such electrodes for delivery of pacingpulses are generally constructed with dimensions to define an optimalsurface area that enables delivery of sufficiently high current densityassociated with the low voltage levels characterized by pacing pulses.One or more vectors, defined via a combination of any one or more of thering or tip electrodes may be established as a primary vector(s) fordelivery of pacing therapy. In the event of multiple cathode/anodeelectrode combinations, a hierarchical relationship may be establishedbetween the two or more primary vectors. The relationship may take intoaccount such factors as the location of each electrode vis-à-visefficacy of therapy delivered through the electrode, the likelihood ofphrenic nerve stimulation, capture threshold and other factors that maybe measured when evaluating pacing efficacy.

During the course of operation of therapy system 10, the lead system ismonitored to identify an occurrence of a lead-related conditionassociated with the primary vector (402). As previously noted,lead-related conditions are typically associated with the leadconductor(s) and breaches in insulation but may also extend to theelectrode and electrode-to-conductor interface. Several approaches formonitoring lead-related conditions have been described in the art andthis disclosure is not limited to any given monitoring technique; anydetection technique may be utilized consistent with the embodiments ofthis disclosure. As a non-limiting example, such techniques include leadimpedance, capture management and capture threshold amplitude, phrenicnerve thresholds, and R-wave amplitudes.

In response to detecting a lead-related condition, the processdetermines whether another primary vector is available for continuedtherapy delivery (404). Such an additional primary vector may be definedby a different set of ring and/or tip electrodes 40, 42, 44, 46, 48 and50. In determining whether an additional primary vector is available,the therapy system will also assess whether the vector is viable. If theadditional primary vector is available and viable, the therapy systemmay switch to that primary vector in response to the detectedlead-related condition (406). The system may also activate an alert toinform the patient and/or clinician of the detected lead-relatedcondition and the action that has been taken (408). However, if there isno remaining primary vector that would be appropriate for providing asuitable electrode combination, an alarm may be activated to alert thepatient and/or clinician (410). The alerts generated at steps 408 and410 may be different. For instance, a lower level alert may be generatedat 408 while a higher level is issued at 410.

In accordance with embodiments of the disclosure, the process assesseswhether the lead system includes an elongated electrode (412). Oneembodiment for assessment of the availability of the elongated electrodeis described with reference to FIGS. 9 and 10. Another example for theidentification is illustrated in U.S. Pat. No. 5,534,018 which isincorporated by reference herein in relevant part. The elongatedelectrode represents an alternative pacing vector which may sustaintherapy delivery albeit with a lower cardiac efficacy in comparison tothe primary vector(s). Nevertheless, the alternative pacing vector mayensure that life sustaining therapy is still provided to the patientuntil such time when a lead replacement may be performed to replace thering or tip electrodes.

If an alternate electrode is available, the process reconfigures theimplantable device for therapy delivery via the alternate vector (414).Unlike switching between the plurality of primary vectors, it iscontemplated that the operation of therapy system 10 is reconfigured ifthe electrode combination includes the elongated electrode. Among otherthings, the reconfiguration of the operation of system 10 compensatesfor the differences in construction of the elongated electrode incomparison to the ring and tip electrode. When utilizing the elongatedelectrode for pacing, the electric fields for pacing are focused on thesecond section (or distal portion) of the electrode thereby reducing thepacing threshold as compared to pacing with the entire surface area ofthe elongated electrode. As discussed elsewhere in this disclosure, theelongated electrode includes first and second sections each having adifferent material. Owing to the markedly different properties of thetwo materials the electric fields of the therapy delivered through theelongated electrode may be configured to be delivered only through thedistal portion. In some embodiments, the pacing pulse waveform generatormay be reconfigured to define a stimulation pulse waveform that issuitable for delivery of pacing pulses through the second section of theelectrode. In any event, the therapy is reconfigured to enable therapydelivery through only the second section which reduces the current drainfor effective therapy delivery.

The inventors have also observed that the timing relationship forsensing of electrical activity representing a cardiac event may beoffset when sensing functionality is switched to the elongatedelectrode. In other words, a single signal representing a cardiac eventthat is detected by the elongated electrode may be received by the sensesignal as a composite signal having two different components. The firstcomponent is the signal received at the distal portion and the secondcomponent is the signal received at the proximal portion.

Because of the lack of a 1 to 1 correspondence between the signal sensedat the distal portion and the sensed at the proximal portion, the sensecircuit may inadvertently interpret the composite signal as twodifferently occurring events. To avoid the erroneous interpretation ofthe composite signal, the IMD may reconfigure the sense circuit inresponse to switching the electrode configuration from the firstelectrode to the alternate electrode (416). In another embodiment,reconfiguration may include adjusting the gain parameter of the sensingcircuit to prevent or significantly reduce sensing of noise signals thatmay result in an erroneous determination of a cardiac event.

With continued reference to FIG. 8, the process may, in an embodiment,switch the therapy modes or pacing configuration of the electrodes foroptimal therapy delivery (418). For example, operation of system 10 maybe switched to a ventricular inhibited (VDD) pacing mode or to aunipolar pacing configuration utilizing the elongated electrode and thecan electrode. The process further switches the cathode selection fromthe primary (ring or tip) electrode to the elongated electrode. As aresult of the operational switch, the therapy energy generated by thedevice is delivered via the vector defined by elongated electrode(cathode) and the can electrode (anode).

In other embodiments, the process may further include adjustments to thepacing parameters to optimize therapy delivery under the alternatevector (420). In one example, a pacing (capture) threshold test may beperformed to re-determine the capture threshold to facilitate optimalpower consumption. In another example, the programmed safety margin maybe assessed to determine whether appropriate capture occurs for thealternate vector. In the event that the capture threshold and/or thesafety margin are inadequate, the process may adjust these parameters toensure capture for therapy delivered under the alternate vector.

Upon completion of the system 10 reconfiguration, the process proceedsto deliver therapy via the alternative pacing vector defined by theelongated electrode and the can electrode.

FIG. 9 illustrates a flow chart in accordance with an embodiment forassessing whether a lead system includes an elongate electrode of thetype contemplated in this disclosure. Specifically, the elongateelectrode is one such as that discussed in FIGS. 5A-B that has a firstsection having a first material and a second section having a secondmaterial.

As noted in FIG. 8 above (412), a determination may be made of whetheran elongate electrode such as electrode 62 is available. In theassessment of FIG. 9, the determination is accomplished through thedelivery of sub-threshold voltage pulses of opposite polarities on thelead conductive pathway such that the impedances observed followingdelivery of the respective opposite polarity pulses can be evaluated.

To this end, stimulation generator 84 (FIG. 4) includes circuitry forgenerating the small sub-threshold voltage pulses, which areperiodically and sequentially issued along the conductive pathway of thelead. An illustration of one example of the sub-threshold voltage pulsewaveform generated by stimulation generator 84 is shown in FIG. 10. Asshown in FIG. 10, sub-threshold voltage pulse 500, 510 are biphasicpulses. Sub-threshold pulse 500 has a leading positive phase 502 whilesub-threshold pulse 510 has a leading negative phase 512. Bothsub-threshold pulses 500, 502 have a peak amplitude of about one (1)volt, and a duration of approximately 0.1 milliseconds. It is believedthat the implementation of circuitry for generating pulses such asdepicted in FIG. 10 would be a matter of routine engineering to those ofordinary skill in the art; therefore, the details of such circuitry willnot be described further herein.

With continued reference to FIG. 9, a sub-threshold voltage pulse of afirst polarity is delivered through the conductive pathway of a givenlead (440). The sub-threshold voltage pulse may be a bi-phasic voltagepulse or a mono-phasic voltage pulse. The first polarity may be either apositive or negative polarity. In response to delivery of the pulse, afirst measurement of a first electrical parameter may be performed(442). Examples of the first electrical parameter that may be measuredinclude a current value of the sub-threshold pulse delivered in theconductive pathway of the lead under test or a voltage value of thevoltage induced in the conductive pathway of the lead under test. Themeasurement may be a unipolar measurement, i.e., electrode to devicehousing or a bipolar measurement through two electrodes of the lead. Thefirst measurement of the electrical parameter subsequent to delivery ofthe sub-threshold pulse of the first polarity represents a firstmeasurement cycle. The first measurement cycle may include one or morepulses being delivered with corresponding impedance measurements foreach delivered pulse.

As noted above, when a sub-threshold pulse is delivered, an electricalparameter on the path is measured. To this end, sensing module 86 (FIG.4) includes circuitry for obtaining samples of the electrical propertyon the path. Other examples of the circuitry are described in commonlyassigned U.S. Pat. Nos. 5,755,742 and 7,233,825, both of which areincorporated herein by reference in their relevant parts. In the presentembodiment of the disclosure, the sampling rate is programmable. Forexample, the peak-to-peak voltage across the heart may be sampled or thecurrent through the electrode to housing can obtained for themeasurement cycle.

Subsequently, a second measurement cycle may be performed with a secondsub-threshold voltage pulse having a second polarity (444). Similar tothe first sub-threshold voltage pulse, the second sub-threshold voltagepulse can be either a bi-phasic or mono-phasic pulse. However, it ispreferred that the second sub-threshold voltage pulse of the secondmeasurement cycle have a profile similar to that of the firstmeasurement cycle in all respects with the exception of the polarity;the second polarity is opposite to that of the first polarity. In otherwords, if the first sub-threshold voltage pulse is a bi-phasic pulsewith a positive polarity, then the second sub-threshold voltage pulsewill be a bi-phasic pulse with a negative polarity and vice versa.

A measurement of a second electrical parameter may be performedfollowing delivery of the second sub-threshold pulse (446). In oneembodiment, the second electrical parameter measured following thesecond sub-threshold pulse may be the same parameter as the parametermeasured following the first sub-threshold pulse at step 442. In anotherembodiment, yet another electrical parameter may be derived from each ofthe first measured electrical parameter and the second measuredelectrical parameter (448). For example, an impedance value may bederived from each of the electrical parameter obtained in the firstmeasurement cycle and the electrical parameter obtained in the secondmeasurement cycle. For ease of illustration, the electrical parameterderived from the first electrical parameter measured in the firstmeasurement cycle will also be referred to as a first electricalparameter in the subsequent steps of the flow chart of FIG. 9.Similarly, the electrical parameter derived from the second electricalparameter measured in the second measurement cycle will also be referredto as a second electrical parameter in the subsequent steps of the flowchart.

The first electrical parameter and the second electrical parameter arecompared to determine whether there is a difference in the values and ifso whether the difference exceeds a predetermined threshold (450). Thecomparison of the electrical parameters for measurement of thedifference between the values of the first electrical parameter and thesecond electrical parameter may be performed by electrical in the IMD 16such as the processor 80 or dedicated circuitry such as impedancemeasurement circuitry. For example, a typical lead having an elongatedelectrode, such as electrode 62 of the present disclosure, will yield animpedance value of about 100 ohms from the positive polarity pulse testwhereas the impedance from the negative polarity pulse test for the sameelectrode-bearing lead is about 60 ohms Therefore, the difference inpolarity for a first and second measurement cycle for that example wouldbe about 40 ohms.

In accordance with the embodiment of FIG. 9, if the difference betweenthe electrical parameter obtained from the first measurement cycle andthe electrical parameter obtained from the second measurement cycleexceeds a threshold value (452), the lead is determined to have anelongated electrode, such as electrode 62 described in FIGS. 5A-B, thathas a first section comprising a first material and a second sectioncomprising a second material that is different from the first material(454). Otherwise, the lead is determined not to have such an elongatedelectrode (456).

The above detailed description includes references to the accompanyingdrawings, which form a part of the detailed description. The drawingsshow, by way of illustration, specific embodiments in which thedisclosure can be practiced. These embodiments are also referred toherein as “examples.” All publications, patents, and patent documentsreferred to in this document are incorporated by reference herein intheir entirety, as though individually incorporated by reference. In theevent of inconsistent usages between this document and those documentsso incorporated by reference, the usage in the incorporated reference(s)should be considered supplementary to that of this document; forirreconcilable inconsistencies, the usage in this document controls.

In this document, the terms “a” or “an” are used, as is common in patentdocuments, to include one or more than one, independent of any otherinstances or usages of “at least one” or “one or more.” In thisdocument, the terms “including” and “in which” are used as theplain-English equivalents of the respective terms “comprising” and“wherein.” Also, in the document, the terms “including” and “comprising”are open-ended, that is, a system, device, article, or process thatincludes elements in addition to those listed after such a term in aclaim are still deemed to fall within the scope of that claim. Moreover,in the document, the terms “first,” “second,” and “third,” etc. are usedmerely as labels, and are not intended to impose numerical requirementson their objects, unless otherwise denoted.

Method examples described herein can be machine or computer-implementedat least in part. Some examples can include a computer-readable mediumor machine-readable medium encoded with instructions operable toconfigure an electronic device to perform methods as described in theabove examples. An implementation of such methods can include code, suchas microcode, assembly language code, a higher-level language code, orthe like. Such code can include computer readable instructions forperforming various methods. The code may form portions of computerprogram products. Further, the code may be tangibly stored on one ormore volatile or non-volatile computer-readable media during executionor at other times. These computer-readable media may include, but arenot limited to, hard disks, removable magnetic disks, removable opticaldisks (e.g., compact disks and digital video disks), magnetic cassettes,memory cards or sticks, random access memories (RAM's), read onlymemories (ROM's), non-volatile random access memory (NVRAM),electrically erasable programmable read-only memory (EEPROM), FLASHmemory, magnetic data storage media, optical data storage media, or thelike.

The term “processor” or “processing circuitry” may generally refer toany of the foregoing logic circuitry, alone or in combination with otherlogic circuitry, or any other equivalent circuitry. Further, the presentdisclosure is not limited in scope to implantable medical devicesincluding only a single processor but may be applicable tomultiple-processor devices as well.

The above description is intended to be illustrative, and notrestrictive. For example, the above-described examples (or one or moreaspects thereof) may be used in combination with each other. Otherembodiments can be used, such as by one of ordinary skill in the artupon reviewing the above description. The Abstract is provided to complywith 37 C.F.R. § 1.72(b), to allow the reader to quickly ascertain thenature of the technical disclosure. It is submitted with theunderstanding that it will not be used to interpret or limit the scopeor meaning of the claims. Also, in the above Detailed Description,various features may be grouped together to streamline the disclosure.This should not be interpreted as intending that an unclaimed disclosedfeature is essential to any claim. Rather, inventive subject matter maylie in less than all features of a particular disclosed embodiment.Thus, the following claims are hereby incorporated into the DetailedDescription, with each claim standing on its own as a separateembodiment. It should also be understood that various changes can bemade in the function and arrangement of elements without departing fromthe scope of the disclosure as set forth in the appended claims and thelegal equivalents thereof.

What is claimed is:
 1. An implantable medical system, comprising: animplantable medical device; and a medical electrical lead system coupledto the implantable medical device, including: a first electrode; and analternate electrode having a first section and a second section, whereinthe implantable medical device delivers therapy through a first vectorincluding the first electrode and reconfigures the therapy for deliverythrough a second vector including the second section of the alternateelectrode but not the first section of the alternate electrode inresponse to detecting a lead-related condition.
 2. The implantablemedical system of claim 1, wherein the first section is a proximalportion and the second section is a distal portion.
 3. The implantablemedical system of claim 1, wherein the second section comprises a firstplurality of segments interspersed between the first section.
 4. Theimplantable medical system of claim 1, further comprising: a sensingcircuit; and an oversense reduction circuit coupled to the sensingcircuit, wherein the oversense reduction circuit analyzes a compositesignal detected through the second vector.
 5. The implantable medicalsystem of claim 1, wherein reconfiguring the therapy to be delivered bythe implantable medical device comprises configuring the alternateelectrode as a cathode.
 6. The implantable medical system of claim 1,wherein reconfiguring the therapy to be delivered by the implantablemedical device comprises switching from a first pacing mode to a secondpacing mode in response to the lead condition.
 7. The implantablemedical system of claim 6, wherein the first pacing mode is a bipolarpacing configuration and the second pacing mode is a unipolar pacingconfiguration.
 8. The implantable medical system of claim 1, whereinreconfiguring the therapy comprises modifying the stimulation profile ofthe pacing pulse.
 9. The implantable medical system of claim 1, whereinreconfiguring the therapy comprises changing the energy threshold of thepacing pulse.
 10. The implantable medical system of claim 1, furthercomprising a sensing circuit, wherein the sensing function isreconfigured in response to the lead related condition.
 11. Theimplantable medical system of claim 10, wherein reconfiguring thesensing circuit comprises adjusting the filter characteristics of thesensing circuit responsive to detecting the lead-related condition. 12.The implantable medical system of claim 10, wherein reconfiguring thesensing circuit comprises adjusting at least one of a threshold and gainparameter of the sensing circuit responsive to detecting thelead-related condition.
 13. The implantable medical system of claim 1,wherein the first section of the alternate electrode comprises a firstmaterial and the second section of the alternate electrode comprises asecond material.
 14. The implantable medical system of claim 1, whereinthe alternate electrode is an elongated electrode.
 15. The implantablemedical system of claim 1, wherein the alternate electrode isconstructed such that the first section comprises approximately 65% toapproximately 95% of the total surface area of the alternate electrode.16. The implantable medical system of claim 1, wherein the alternateelectrode is constructed such that the first section comprisesapproximately 80% to approximately 90% of the total surface area of thealternate electrode.
 17. The implantable medical system of claim 1,wherein the first electrode is coupled to a stimulation generator withinthe implantable medical device by a first conductor, and further whereinthe alternate electrode is coupled to the stimulation generator withinthe implantable medical device by a second conductor to enable couplingof defibrillation energy to and through both the first and secondsections of the alternate electrode.
 18. An implantable medical system,comprising: an implantable medical device; and a medical electrical leadsystem coupled to the implantable medical device, comprising: a firstelectrode of a first pacing vector; and an alternate electrode having afirst section and a second section of a cardioversion/defibrillationvector through which at least one of cardioversion and defibrillationtherapy is deliverable, wherein the at least one of the cardioversionand defibrillation therapy is delivered through both the first andsecond sections of the alternate electrode, wherein the implantablemedical device delivers therapy through the first pacing vectorincluding the first electrode and reconfigures the therapy for deliverythrough a second pacing vector including the second section of thealternate electrode but not the first section of the alternate electrodein response to detecting a lead-related condition.
 19. The implantablemedical system of claim 18, wherein reconfiguring the therapy to bedelivered by the implantable medical device comprises switching from afirst pacing mode to a second pacing mode in response detecting alead-related condition, wherein the first pacing mode is a bipolarpacing configuration and the second pacing mode is a unipolar pacingconfiguration.
 20. The implantable medical system of claim 18, whereinthe alternate electrode is constructed such that the first sectioncomprises approximately 65% to approximately 95% of the total surfacearea of the alternate electrode.